Incorporating ionomers within highly porous, dimensionally-stable polymer electrolyte membrane (PEM) substrates increases the performance and longevity of PEM devices, such as fuel cells and electrolyzers. GORE-TEX® expanded polytetrafluoroethylene (PTFE) is an example of a commonly-used polymer electrolyte membrane substrate containing a fluorinated, microporous structure. However, GORE-TEX® expanded PTFE may have non-uniform porosity, through-holes that are not well-defined, and a low degree of porosity, which negatively impact fuel cell and electrolyzer performance. Aluminum oxide membranes are another example of dimensionally-stable polymer electrolyte membrane substrates with high porosity and straight-through pores, but they are often brittle, susceptible to crack formation under stress, and difficult to apply in a continuous, roll-to-roll type manufacturing process. The thickness of aluminum oxide membranes is also 100 microns or higher, thus making them impractical for applications that require ultra-thin (˜25 micron or less) membrane substrates.
Some previous attempts have been made to fabricate membranes with patterned and highly dense pores using thermal processing methods (see, for example, U.S. Pat. No. 7,708,544 B2, inventor Pricone, issued May 4, 2010; Schift et al., “Perforated polymer membranes fabricated by nanoimprint,” Microelectronic Engineering, 83 (4-9):873-875 (2006); Shibata et al., “Modified imprinting process using hollow microneedle array for forming through holes in polymers,” Microelectronic Engineering, 88(8):2121-2125 (2011); and Yanagishita et al., “Polymer through-hole membrane fabricated by nanoimprinting using metal molds with high aspect ratios,” Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 25 (4):L35 (2007), all of which are incorporated herein by reference). Membranes using straight-through pores produced by pulsed laser drilling have also been fabricated for use in fuel cell or electrolyzer applications (see, for example, U.S. Pat. No. 7,521,144 B2, inventors Shimohira et al., issued Apr. 21, 2009; and U.S. Patent Application Publication No. US 2009/0253016 A1, inventor Katayama, published Oct. 8, 2009, both of which are incorporated herein by reference). Another method to form pores in membrane substrates is the use of photolithography wherein a photoresist is cast on a material, followed by directing UV-light over a patterned mask in order to develop select areas. Upon exposure of the non-masked (unprotected) areas to chemicals, the material is chemically etched until the desired porosity is obtained. However, both chemical etching and optical ablation (e.g. laser drilling) methods often suffer from severely tapered pores and are also difficult to adapt to continuous and rapid roll-to-roll manufacturing processes. Additionally, optical ablation methods often experience an inability to focus the laser uniformly through the entire film thickness. Laser drilling is also limited to aromatic polymers, as the focused light from the laser will not be absorbed in sufficient amounts by non-aromatic polymers with the light simply passing through the material.
Other documents that may be of interest include U.S. Pat. No. 7,947,405 B2, inventors Mittelsteadt et al., issued May 24, 2011; U.S. Pat. No. 7,867,669 B2, inventors Liu et al, issued Jan. 11, 2011; and Heckele et al., “Review on micro molding of thermoplastic polymers,” Journal of Micromechanics and Microengineering, 14 (3):R1-R14 (2004), all of which are incorporated herein by reference.